# Using custom structs

This tutorial shows how to use custom structs on the GPU. Our example will be a one dimensional interpolation. Lets start with the CPU version:

using CUDA

struct Interpolate{A}
xs::A
ys::A
end

function (itp::Interpolate)(x)
i = searchsortedfirst(itp.xs, x)
i = clamp(i, firstindex(itp.ys), lastindex(itp.ys))
@inbounds itp.ys[i]
end

xs_cpu = [1.0, 2.0, 3.0]
ys_cpu = [10.0,20.0,30.0]
itp_cpu = Interpolate(xs_cpu, ys_cpu)
pts_cpu = [1.1,2.3]
result_cpu = itp_cpu.(pts_cpu)
2-element Vector{Float64}:
20.0
30.0

Ok the CPU code works, let's move our data to the GPU:

itp = Interpolate(CuArray(xs_cpu), CuArray(ys_cpu))
pts = CuArray(pts_cpu);

If we try to call our interpolate itp.(pts), we get an error however:

...
KernelError: passing and using non-bitstype argument
...

Why does it throw an error? Our calculation involves a custom type Interpolate{CuArray{Float64, 1}}. At the end of the day all arguments of a CUDA kernel need to be bitstypes. However we have

isbitstype(typeof(itp))
false

How to fix this? The answer is, that there is a conversion mechanism, which adapts objects into CUDA compatible bitstypes. It is based on the Adapt.jl package and basic types like CuArray already participate in this mechanism. For custom types, we just need to add a conversion rule like so:

import Adapt
Interpolate(xs, ys)
end

Now our struct plays nicely with CUDA.jl:

result = itp.(pts)
2-element CuArray{Float64, 1, CUDA.Mem.DeviceBuffer}:
20.0
30.0

It works, we get the same result as on the CPU.

@assert CuArray(result_cpu) == result

Alternatively instead of defining Adapt.adapt_structure explictly, we could have done

Adapt.@adapt_structure Interpolate

which expands to the same code that we wrote manually.